Hydrogen production by catalytic coal gasification

ABSTRACT

Coal is catalytically reacted with steam to produce hydrogen. Various Group I metal salts such as K 2  CO 3 , Na 2  CO 3  and borax are used as catalysts. These catalysts are stabilized with fluoride containing salts such as CaF 2  to thereby extend their life. Alternatively, NaF was found to be a thermally stable catalyst for the reaction.

BACKGROUND OF THE INVENTION

This invention relates to a process for gasifying coal using steam,lime, and stabilized Group I metal salt catalysts. More specifically,the invention also concerns stabilized Group I metal salt catalystsusing fluoride salts of Group II metals such as CaF₂ for catalyststabilization.

Early work involving the production of hydrogen from coal has beendisclosed in British Patent 8743 (1910). This patent showed a reactionwith coal char and steam to produce hydrogen and CO₂. The reactionmixture contained lime as a carbon oxide acceptor and an alkali saltcatalyst such as K₂ CO₃, borax, Na₂ CO₃, etc. For convenience, thisprocess is termed the catalytic-steam-carbon-lime process (CSCL).

Other techniques for producing hydrogen are known and include thesteam-oxygen processes which basically produce a synthesis gas composedmainly of carbon monoxide and hydrogen. This is achieved by a hightemperature endothermic reaction of steam with carbon (H₂ O + C → CO +H₂) and the exothermic reaction of oxygen with carbon (C + 1/2 O₂ → CO).The synthesis gas produced must be catalytically shifted to hydrogen andCO₂ through the lower temperature water gas shift reaction (CO + H₂ O →CO₂ + H₂). Carbon dioxide and sulfur gases are then removed from the gasstream by liquid phase absorption to yield relatively pure hydrogen gas.Lurgi, Koppers-Totzek and Winkler type gasifiers could all be used forthese processes to produce hydrogen.

One major disadvantage of the steam-oxygen processes is that thesteam-oxygen gasifiers operate at very high temperature (3500° F in thecase of Koppers-Totzek) in comparison to the CSCL process which employsa reaction temperature of about 1200° F - 1400° F.

Furthermore, oxygen is required in the gasifiers to provide the energyrequired for the endothermic steam-carbon reaction. In the case ofKoppers-Totzek, approximately 0.8 tons of oxygen is required for everyton of coal gasified. By comparison, no oxygen is required in the CSCLprocess.

Finally, separate water-gas, shift reactor and CO₂ and sulfur gasremoval is required for the steam-oxygen processes. By comparison,sulfur and CO₂ removal takes place "in-situ" in the CSCL process.

Another technique for producing hydrogen is by the steam-iron processwhich produces hydrogen through the reaction of steam with iron (3Fe +4H₂ O → Fe₃ O₄ + 4H₂). The iron is then regenerated by reduction of theiron oxide product with a clean synthesis gas (CO + H₂). The primarydisadvantage of this scheme in comparison with the CSCL process is that1/2 - 3/4 of a mole of clean (sulfur free) impure hydrogen (CO + H₂) isrequired to regenerate the iron oxide for every mole of pure hydrogenwhich is produced. On the other hand low grade, high ash residual charcan be used to regenerate the lime in the CO₂ acceptor scheme.

A basic problem, however, exists with the CSCL process. This probleminvolves the deactivation of the Group I metal salt catalyst which tendsto be poisoned by reaction at high temperature with silica, alumina oralumino-silicates (in the coal ash) in the gasifier or in the limeregenerator. British Patent 8743 (1910) referred to, supra, did notteach how to stabilize the catalyst to prevent deactivation or how toregenerate the lime.

It is, therefore, an object of this invention to provide a CSCL processand apparatus for producing hydrogen in which the lime is regeneratedand the catalyst remains active and, hence, reusable.

Another object is to provide a CSCL process and apparatus for producinghydrogen rapidlyat 1200° F - 1400° F via the reaction: ##STR1##

Another object is to provide a CSCL process and apparatus having highreaction selectivity to hydrogen, with carbon dioxide being the onlyother significant reaction product. A sufficiently low reactiontemperature would permit most of the carbon monoxide initially formed tobe shifted through a reaction with steam (CO + H₂ O → CO₂ + H₂) tocarbon dioxide and sorbed by the lime.

Another object is to provide a CSCL process and apparatus wherein thehydrogen production reaction is slightly exothermic and, hence, norequirement is necessary for heat addition (or oxygen addition) in thegasifier.

Another object is to provide a CSCL process and apparatus having a limeregeneration reaction ##STR2## which can be completely decoupled fromthe hydrogen production reaction. This enables use of a variety of lowcost energy sources (e.g., combustion of the residual high ash contentchar with air rather than oxygen) to effect the limestone decomposition.

Another object is to provide a CSCL process having a reactive system inwhich sulfur compounds which are present in coal are retained in thelime/limestone mixtures thereby eliminating the necessity for a separatesulfur removal system.

Another object is to provide a CSCL process and apparatus having areaction system which is capable of recycling stabilized catalysts manytimes between a primary reactor and a lime regenerator without losingtheir activity.

Another object is to provide new and improved catalysts which may beemployed in the CSCL process wherein the catalysts are able to: 1)gasify less reactive chars in addition to the highly reactive lignitesthat are used primarily in the present systems; and 2) operate at lowertemperatures thereby resulting in greater retention of both carbonoxides and sulfur compounds in the lime without coking or fusionproblems.

THE INVENTION

According to the invention, new and novel catalysts and stabilizerstherefor are provided for the CSCL process to produce hydrogen. Thecatalyst comprises either:

a. a Group I metal salt and a fluoride salt of a Group II metal, or

b. Group I metal fluoride salt such as sodium fluoride.

Use of the Group II metal fluoride salt appears to stabilize thecatalytic activity of the Group I metal salt and permits its reuseseveral times in the process, including lime regeneration ##STR3## Bycontrast, if the stabilizer is not employed, the activity of the alkalisalt catalyst is only effective for one reaction; this is probablydependent on the silica or alumina content. In general, catalystactivity decreases less rapidly with increasing CaF₂ content, while theactivity loss rate depends on the SiO₂ content.

Typical Group I metal salt catalysts which may be employed in theprocess include: K₂ CO₃, K₂ B₄ O₇, Na₂ B₄ O₇, Na₂ CO₃ and mixtures ofGroup I metal salts; a NaCl-KCl mixture has been found to havesignificantly higher activity than the individual components. Also,these catalysts in their natural mineral forms such as sylvinite, sodaash, kernite, trona and borax, have good activity.

Stabilizers include CaF₂, in pure form and in the mineral form such asfluorite. It is possible that the fluorosilicates of Ca, Na and Ktogether with their mineral analogues may also be suitable.

Typical stabilized catalyst systems include:

K₂ co₃ --caF₂, Na₂ B₄ O₇ --CaF₂, Na₂ CO₃.NaHCO₃ --CaF₂, NaCl--CaF₂, etc.and NaF (which is stable by itself).

Table 1 shows the results of employing a stabilizer such as CaF₂ with analkali catalyst such as K₂ CO₃. It will be evident that Samples D and Eperformed significantly better than the other catalyst combinations.Sample F containing substantial quantities of silica resulted in areduction of catalyst activity and appears to confirm the poisoningeffect on the catalyst. Sample H demonstrates the stability, by itself,of a NaF catalyst.

The addition of more CaF₂ is postulated to cause the in situ formationof a catalytically active fluorosilicate complex.

                                      TABLE 1                                     __________________________________________________________________________    Catalyst Recycle Experiments                                                                       Regeneration                                                                         Gasification Rate                                                      Temperature                                                                          % Carbon Gasified                                 Sample           Cycle*                                                                            ° C                                                                           Hour at 650° C                                                                   Remarks                                 __________________________________________________________________________    Lime-char + 5% K.sub.2 CO.sub.3                                                                1   --     100       New char added after each cycle on                                            the                                                      2   850    52        basis that all residual carbon was      A                3   850     3        burned out during regeneration. No                       4   850     0        additional catalyst or lime was                                               added.                                  Lime-Char + 5% Na.sub.2 B.sub.4 O.sub.7                                                        1   --     53        New char added after each cycle on                                            the                                                      2   850    11        basis that all residual carbon was      B                3   850     0        burned out during regeneration. No                                            additional catalyst or lime was                                               added.                                  Lime-Char + 5% KF                                                                              1   --     85        New char added after each cycle on                                            the                                                      2   850    28        basis that all residual carbon was      C                3   850     6        burned out during regeneration. No                                            additional catalyst or lime was                                               added.                                  Lime-Char + 5% K.sub.2 CO.sub.3 +                                                              1   --     89        New char added after each cycle on                                            the                                                      2   850    88        basis that all residual carbon was      10% CaF.sub.2    3   850    92        burned out during regeneration. No                       4   850    97        additional catalyst or lime was                                               added.                                  D                5   900    82                                                                 6   900    83                                                                 7   900    25                                                                 8   900     6                                                Lime-Char + 5% K.sub.2 CO.sub.3 +                                                              1   --     88                                                                 2   850    90        New char added after each cycle.        3% CaF.sub.2     3   850    66                                                                 4   850     7        3% fresh CaF.sub.2 added after                                                Cycle 4.                                E                5   850    56                                                                 6   850    46                                                                 7   850    46                                                                 8   850    40                                                                 9   850    42                                                                 10  850    35                                                                 11  850    33                                                Lime-Char + 5% K.sub.2 CO.sub.3 +                                                              1   --     80                                                                 2   850    65        New char added after each cycle.        5% CaF.sub.2 +   3   850    95                                                F                4   850    110                                               3% SiO.sub.2     5   850    22        3% fresh CaF.sub.2 added after                                                Cycle 5.                                                 6   850    90                                                Lime-Charcoal + 5% Na.sub.2 B.sub.4 O.sub.7 +                                                  1   --     14                                                                 2   850     5        New char added after each cycle.        G 5% SiO.sub.2   3   850     2                                                3% CaF.sub.2 Added After Cycle 3                                                               4   850    27        3% fresh CaF.sub.2 added after                                                Cycle 3.                                                 5   850    34                                                Lime-Charcoal + 5% NaF                                                                         1   --     52                                                                 2   850    47        New char added after each cycle.        H                3   850    48                                                                 4   850    49                                                                 5   850    53                                                __________________________________________________________________________     *Cycle 1 in all cases refers to the initial reaction rate prior to            exposure to the lime regeneration conditions.                            

Weight ratios of catalyst-stabilizer combinations may vary from about0.2 to 5 and typical ratios are:

K₂ CO₃ --CaF₂ 1:1, K₂ CO₃ --CaF₂ 2:1; K₂ CO₃ --CaF₂ 1:2;

Borax--CaF₂ 1:1; Borax--CaF₂ 1:2; Borax--CaF₂ 2:1;

NaCl--CaF₂ 2:1; Trona--CaF₂ 1:1; Trona--CaF₂ 1:3.

Types of CaO acceptors include: decomposed, chemically pure, calcite;run-of-mine limestone; dolomite; etc. A useful weight range of limeacceptor varies from about 10% to 90% by weight.

The type of carbonaceous materials which may be employed in thisinvention include: char, coal, cellulosic wastes, agricultural wastes,etc.

The catalyst may be dry blended with the char, lime coal, etc., prior toreaction; alternately, the char or lime, may be impregnated with thecatalyst prior to gasification. Concentrations of catalyst in the charor lime may vary from about 0.5% to 25%. The catalyst may also beemployed as compressed pellets, sintered pellets, sintered compressedpellets, etc.

IN THE DRAWINGS

FIG. 1 shows a schematic view of an apparatus which may be employed forcarrying out the process of this invention;

FIG. 2 is a graph showing the relation between reaction time andconversion rate for various mixtures of catalyst both dry blended andwet impregnated;

FIG. 3 is a graph showing typical product gas compositions and H₂ :CO₂ratios as a function of time in a batch reactor run.

FIG. 4 is a graph showing the relation between CO₂ conversion andreaction time for a catalyzed and uncatalyzed reaction; and

FIG. 5 is a graph showing the relation between carbon gasification andreaction time for various catalyst types.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One form of apparatus which may be employed to carry out the process ofthis invention is shown schematically in FIG 1.

This provides a reactor 10 for converting H₂ O, char and lime into CaCO₃and H₂ as follows: ##STR4##

A regenerator 11 converts the calcium carbonate to lime and reacts charand air to form CO₂ and produce process steam in a waste heat boiler asfollows: ##STR5##

Char for the process may be stored in feed hoppers 12, 13 and fedthrough a lime 14 into the reactor using a steam carrier as shown.

The catalyst may be dry or wet blended with the char or lime prior tofeeding into the reactor. Alternately, as shown, the catalyst may be fedfrom a feed tank 15 into the reactor 10 and applied to the char or limefrom a plurality of sprayers 16. The stabilizer such as CaF₂ may be fedinto the reactor as a slurry along with the catalyst solution.Alternately, the stabilizer may be fed into the reactor along with thechar. As a third possibility, the stabilizer may be fed into the systemalong with make-up CaCO₃ through line 17.

Reactor temperatures may vary from about 550° C - 800° C and pressuresmay vary from at least above 1 atmosphere and preferably about 1 - 20atmospheres. Process steam is fed into the reactor bottom through line18 to provide water for the reaction and also to produce a fluidizedbed.

Steam and hydrogen are removed together as overhead through line 19.Some of the process steam is recovered in the waste heat boiler unit 20and hot boiler feed water is produced in condenser 21.

The bottoms from the reaction principally include CaCO₃, char, catalyst,and stabilizer. CaCO₃, being the heaviest and largest particle size, istaken from the lowest part of the reactor through line 25 and fed to theregenerator 11 along with preheated air fed through line 26.

After entering into the catalyzed reaction to produce hydrogen, theresidual char is forwarded into the regenerator 11 through line 27. Inthe regenerator, CaCO₃ is converted to lime and residual char to CO₂ attemperatures of about 800° C - 1100° C; a preferred temperature rangevaries from about 870° C - 980° C. Regenerator pressures of about 1 - 20atmospheres are suitable. The reactor and regenerator are operated atapproximately the same pressures. Catalyst, stabilizer and lime areremoved as overheads and recycled through line 28 to the reactor 10using a steam carrier injected through line 29.

Hot combustion gases, and solids such as coal ash, CO₂, catalyst finesand spent limestone containing sulfur from the burned coal are removedperiodically through line 30 and passed to a separator 35. Hotcombustion gases are removed from the separator 35 and passed throughline 36 to a heat exchanger 37 for production of process steam and thento heat incoming air for use in the regenerator 11. The solids whichhave been separated in separator 35 may be passed to a sludge pond.

FIG. 2 shows the differences between employing the catalyst dry blendedwith the char compared to wet impregnation of the char. In all cases,except 1% K₂ CO₃, use of wet impregnation of the char by the catalystachieves faster gasification rates.

FIG. 3 shows the high purity gas produced by the process of thisinvention since the lime functions as an acceptor to combine with theCO₂ which is produced.

FIG. 4 shows the carbon conversion rate from Egypt Valley coal char at650° C using K₂ CO₃ compared to an uncatalyzed reaction. It is evidentthat no significant H₂ conversion is attainable at these temperatureswithout the catalyst.

FIG. 5 shows a comparison between use of an alkali salt catalyst and anon-alkali salt such as FeCl₃ to achieve gasification. It is obviousthat the FeCl₃ conversion rate is not sufficiently high to render itsuse attractive for a catalyst.

The effect of temperature on gasification rates varies depending on thecatalyst type. This is shown in the following Table 2.

                  TABLE 2                                                         ______________________________________                                        Effect of Temperature on Gasification Rate                                                 Temperature  Reaction Rate                                       Catalyst     ° C   % Conversion Min.sup.-1                             ______________________________________                                        5 wt. % K.sub.2 CO.sub.3                                                                   550          0.061                                                            600          0.18                                                             650          2.7                                                              700          5.4                                                              750          10.8                                                5 wt. % Na.sub.2 B.sub.4 O.sub.7                                                           550          0.070                                                            600          0.16                                                             650          0.89                                                             700          2.1                                                              750          7.7                                                 No Catalyst  650          0.01                                                             750          0.27                                                ______________________________________                                    

The data indicates that K₂ CO₃ is more reactive than Na₂ B₄ O₇ and thatthe minimum effective gasification temperature for both catalysts isabout 650° C. Reaction temperatures will vary for other catalysts, butthe minimum is probably about 550° C.

The catalysts of this invention provide an economic method for hydrogenproduction in the CSCL process. This in turn permits a new and improvedplant and process suitable for continuous operation rather thannecessitating a removal of the entire catalyst load and consequentfrequent interruption in the process.

We have also found that the stabilized catalysts described herein arevery active for classical coal gasification reactions such assteam-carbon as shown in the following Table 3. It can be seen fromTable 3 that the CaF₂ stabilized K₂ CO₃ is several times more activethan pure K₂ CO₃ for steam-carbon reactions. Furthermore, the catalystdoes not sinter at the operating temperatures.

                  TABLE 3                                                         ______________________________________                                                              Gas                                                                           Generation                                                                    Rate For                                                              Temp.   Equivalent   Condition                                  Mixture       ° C                                                                            Carbon cc/min                                                                              of Residue                                 ______________________________________                                        C + STEAM + K.sub.2 CO.sub.3                                                                825° C                                                                         10.2         Sintered                                                                      (Probable -   Melting)                     C + STEAM + K.sub.2 CO.sub.3                                                                825° C                                                                         38.0         Unsintered                                 + CaF.sub.2                                                                   ______________________________________                                    

We claim:
 1. A process for producing hydrogen comprising the steps of:a.reacting steam, carbonaceous material and lime with a catalyst toproduce hydrogen, residual carbon and CaCO₃, the catalyst being selectedfrom the group consisting of (1) NaF and (2) a salt of a Group I metalstabilized with a fluoride of a Group II metal; b. regenerating CaCO₃ tolime, converting the residual char to combustion products; c. reusingthe catalyst and regenerated lime for further reaction with steam andcarbonaceous material to produce additional hydrogen, and d. drawing offproduct, including hydrogen.
 2. The process of claim 1 in which thecatalyst comprises primarily a salt of a Group I metal stabilized withcalcium fluoride.
 3. A process for producing hydrogen by the CSCLprocess comprising the steps of:a. reacting steam, carbonaceous materialand lime with a catalyst in a first reaction zone, said catalyst beingselected from the group consisting of (1) NaF and (2) a salt of a GroupI metal stabilized with a fluoride salt of a Group II metal; b. removingcatalyst, CaCO₃, steam and residual char to a second reaction zone; c.regenerating CaCO₃ to lime and converting residual char to combustionproducts in said second reaction zone; d. recycling the catalyst andregenerated lime to the first reaction zone and e. taking a productstream, including H₂, from the first reaction zone.
 4. The process ofclaim 3 in which the fluoride salt is CaF₂.
 5. The process of claim 3 inwhich the salt of a Group I metal is selected from the class consistingof: (a) K₂ CO₃, K₂ B₄ O₇, Na₂ B₄ O₇, NaCl, KCl, Na₂ CO₃, NaHCO₃,NaCl--KCl, and mixtures thereof, sylvinite, soda ash, kernite, trona,and borax.
 6. The process of claim 3 in which the lime is present in theform of a material selected from the class consisting of: (a)decomposed, chemically pure, calcite; (b) run-of-mine-limestone and (c)dolomite.
 7. The process of claim 3 in which the reaction temperature inthe first reaction zone is within the range from about 550° C - 800° Cand the pressure in the first reaction zone is within the range fromabout one atmosphere to about 20 atmospheres.
 8. The process of claim 3in which the operating pressures in the first and second reaction zonesare substantially equal.
 9. The process of claim 3 in which the firstreaction zone comprises a fluidized reaction bed.
 10. The process ofclaim 3 in which the catalyst and lime are recycled with a steamcarrier.
 11. The process of claim 3 in which the temperature, in thesecond reaction zone, is within the range from about 800° C - 1100° Cand the pressure is in the range from about 1 to 20 atmospheres.
 12. Theprocess of claim 3 in which the temperature, in the second reactionzone, is in the range from about 870° C - 980° C.